In electrostatography an image comprising an electrostatic field pattern, usually of non-uniform strength, (also referred to as an electrostatic latent image) is formed on an insulative surface of an electrostatographic element by any of various methods. For example, the electrostatic latent image may be formed electrophotographically (i.e., by imagewise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on a surface of an electrophotographic element comprising a photoconductive layer and an electrically conductive substrate), or it may be formed by dielectric recording (i.e., by direct electrical formation of an electrostatic field pattern on a surface of dielectric material). Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with charged toner particles. If desired, the toner image can then be transferred to a final support material or receiver such as a web or sheet of paper and affixed thereto to form a permanent record of the original.
Historically, the transfer of toner images between supporting surfaces has been accomplished with the electrostatic transfer of either a corotron or a roller or belt electrode biased to a certain potential, such electrode being referred to as a bias transfer member (roll or belt). In corona-induced transfer as, for example, disclosed by Vandenberg in U.S. Pat. No. 2,836,725, the final support sheet is placed in direct contact with the toner image while the image is supported on the photoconductive surface. The back of the sheet, that is, the side away from the image, is sprayed with a corona discharge having a polarity opposite to that carried by the toner particle causing the toner to be electrostatically transferred to the sheet. The corotron system is relatively simple. The charges deposited electrostatically tack the final support material, such as paper, to the original toner support, such as, the photoconductor, in addition to creating the desired electric field affecting transfer of the toner to the paper. However, the strong attraction between the paper and the original toner support makes it mechanically difficult to separate the two supports.
Transfer of developed images from the photoconductor to the final support material with the aid of a biased transfer member, such as a biased transfer roll, as a means of controlling the forces acting on the toner during transfer and of avoiding the severe tacking problems encountered with the use of the corona induction system have been tried with limited success. A bias transfer member is a member for electrically cooperating with a conductive support surface to attract electrically charged particles from the support surface towards the member. Bias transfer members are well known in the art. A bias transfer roll is disclosed by Fitch in U.S. Pat. No. 2,807,233 where a metal roll coated with a resilient coating having a resistivity of at least 10.sup.6 ohm cm is used as a bias transfer member. Because of the resistivity of the coating, the amount of bias that can be applied to the roll is limited to relatively low operating values because, at the higher ranges, the air in or about the transfer zone begins to break down, i.e., ionizes causing the image to be degraded during transfer. Shelffo, in U.S. Pat. No. 3,520,604, discloses a transfer roll made of a conductive rubber having a resistivity in the range of 10.sup.16 to 10.sup.11 ohm cm. Here, in order to give the roll the needed resiliency required in most practical applications, the coating must be relatively thick. A thick coating of high resistivity acts to build up a surface charge on the roll resulting in air break down in the transfer region and eventually copy degradation.
More recently, improved bias transfer members have been disclosed which reportedly have overcome many of the electrical and image degradation problems associated with some of the previous transfer techniques. Dolcimascolo et al, in U.S. Pat. No. 3,702,482, disclose a multiple layer transfer roll member for transferring xerographic images under controlled conditions. The member is capable of electrically cooperating with a conductive support surface to attract charged toner particles from the support surface towards the member or towards a transfer material such as paper positioned therebetween, the member having a conductive substrate for supporting a biased potential thereon, an intermediate blanket (primary layer) placed in contact with the substrate to the outer periphery of the blanket and a relatively thin outer coating (secondary layer) placed over the blanket layer having an electrical resistivity to minimize ionization of the atmosphere when the transfer member is placed in electrical cooperation with the image support surface and providing a good toner release property enabling the device to be cleaned of the toner. Meagher, in U.S. Pat. No. 3,781,105 discloses a similar transfer member employed in conjunction with a variable electrical bias means for regulating automatically the electrical field levels at various points on the transfer member during the transfer operation and providing constant current control.
In the preferred embodiment, the transfer member disclosed in U.S. Pat. No. 3,702,482 and U.S. Pat. No. 3,781,105 consists of a roller having a central biasable conductive core further having an intermediate blanket or electrically "relaxable" layer (primary layer) surrounding and in electrical contact with the core, and further having a second blanket or electrically "self-leveling" outer layer (secondary layer) surrounding and in electrical contact with the primary layer. Under operating conditions, it is desirable for optimal image transfer to maintain a relatively constant current flow of less than about 30 micro amps in the nip area between the transfer roll surface, transfer material, and photoconductive surface from which a developed image is to be transferred. For this condition to exist at given potentials, the resistivity of the primary and secondary layers must be within critical values and preferably be relatively constant under normally anticipated extremes of operating conditions. Preferably, it has been found that the primary layer should be a resilient elastomeric material having a volume resistivity within the range of 10.sup.7 to less than 10.sup.11 ohm cm, and the secondary layer should also be a resilient material having a volume resistivity within the range of 10.sup.11 to 10.sup.15 ohm cm.
In practice, it has been found that elastomeric materials used in the transfer member such as polyurethanes which exhibit resistivities within the above ranges, or the resistivities of which can be adjusted or controlled to within the above ranges, are moisture sensitive such that the resistivity may vary by as much as a factor of 50 between 10% and 80% relative humidity as a function of the amount of moisture absorbed from or lost to the surrounding atmosphere. For example, in the case of the polyurethane materials which are employed as the primary layer and which have exceptional good electrical characteristics, the volume resistivity may change from 10.sup.11 ohm cm at low moisture contents, i.e., less than about 0.1% moisture, to 10.sup.9 ohm cm at higher moisture levels, i.e., about 2.5% moisture. Other polyurethanes suitable for use as the secondary layer exhibit resistivity variations from about 10.sup.15 to 10.sup.13 ohm cm as a function of increasing moisture content. The consequent variations in resistivity due to relative humidity effects will ordinarily give rise to erratic performance of the transfer member from day to day particularly in terms of transfer efficiency, i.e., the quality of the image transferred unless compensated for by a concomitant change in the voltages sufficient to maintain a constant nip current, as disclosed by Meagher, in U.S. Pat. No. 3,781,105.
Several attempts have been made in the past both to control the resistivity of such materials to within the critical ranges necessary for optimal image transfer and, at the same time, to reduce the moisture sensitivity of such materials to changes in relative humidity so that the resistivity of the materials remains relatively constant within the ranges required for optimal image transfer. For example, Seanor et al, in U.S. Pat. No. 3,959,574, disclose that the resistivity of the elastomeric materials which constitute the primary layers of the multiple layer transfer roll members of Dolcimascolo et al, can be controlled to within the preferred resistivity range of about 10.sup.7 to about 10.sup.11 ohm cm and can be rendered less sensitive to changes in relative humidity by the addition of certain ionic compounds or agents to the elastomeric materials. Particularly preferred additives disclosed by Seanor et al are quaternary ammonium compounds, including tetraheptyl ammonium bromide, trimethyloctadecylammonium chloride, and benzyltrimethylammonium chloride. The additive compounds or agents of Seanor et al are worked into the polyurethane by direct melting of the additive into the polyurethane or by incorporating a solution or dispersion of the additive into the polyurethane. As a result, the additive agents of Seanor et al are not anchored in the elastomeric composition and are leached out of the elastomer over time during normal operations resulting in a decline in the level of conductivity in the polyurethane elastomers.
Chen et al, in U.S. Pat. No. 4,729,925 and U.S. Pat. No. 4,742,941 disclose, as coating materials for biasable transfer members, polyurethane elastomers made from certain polyisocyanate prepolymers and polyols in which the resistivity can be maintained between 1.times.10.sup.9 and 1.times.10.sup.11 ohm cm by copolymerizing with the polyisocyanate prepolymers and polyol hardening compounds used to make the polyurethane elastomers certain polyol charge-control agents formed from certain metal salts complexed with particular polyether diols such as, for example, bis[oxydiethylenebis(polycaprolactone)yl]5-sulfo-1,3-benzenedicarboxylate, methyltriphenylphosphonium salt. Unlike the additive control agents of Seanor et al, the polyol charge-control agents of Chen et al are not prone to being leached out of the elastomer during normal usage since they constitute an integral part of the cured polyurethane elastomer into which they are incorporated by virtue of having been copolymerized with the polyisocyanate prepolymers and polyol components used to make the polyurethane during the preparation of the elastomer. The polyurethane elastomers of Chen et al, however, are moisture sensitive. Reference to curve 2 in FIG. 2 of U.S. Pat. No. 4,729,925, indicates, for example, that the volume resistivity of the conductive polyurethane elastomer of Example 15 prepared from a commercial polyurethane mix and the polyol control agent of Example 10 therein i.e., bis[oxydiethylenebis(polycaprolactone)yl]5-sulfo-1,3-benzenedicarboxylate, methyltriphenylphosphonium salt, decreased by a factor of about 6.5 when the relative humidity changed from 25% to about 85%.
From the foregoing, it can be seen that there still remains a need in the art for a biasable transfer member capable of electrically cooperating with a conductive support surface to attract charged toner particles towards the member or towards a transfer material such as a sheet of paper positioned between the member and the conductive support in which the resistivity not only can be controlled or adjusted to within a specific range necessary for optimal image transfer but also one in which the resistivity is substantially insensitive to widely varying changes in relative humidity encountered during normal operating conditions such that the resistivity remains relatively constant within the range required for optimal image transfer. The present invention provides such a biasable transfer member and methods for making same.